1. Field of the Invention
The present invention relates to an exhaust gas purification apparatus which includes a plurality of different kinds of catalysts arranged in series with each other and which can purify nitrogen oxides (hereinafter, NOx) included in exhaust gas from an internal combustion engine at lean air-fuel ratios at a higher NOx purification rate than the total of the NOx purification rates of the individual catalysts.
2. Description of the Related Art
To improve fuel economy and to suppress exhaust of carbon dioxide gas to thereby reduce global warming, engines capable of fuel combustion at lean air-fuel ratios (lean burn engines) are being developed and are partially in actual use. Since a conventional catalyst (three-way catalyst) cannot reduce and purify NOx at lean air-fuel ratios, there is a need to develop a catalyst or system that can purify NOx even at lean air-fuel ratios.
Japanese Patent Publication HEI 1-139145 proposes a copper (Cu)/zeolite catalyst in which copper is ion-exchanged on a zeolite carrier. This catalyst is capable of purifying NOx at lean air-fuel ratios. However, the NOx purification ability of the Cu/zeolite catalyst is poor at stoichiometric and rich air-fuel ratios. Further, the Cu/zeolite catalyst has little ability to purify hydrocarbons (hereinafter, HC) included in exhaust gas.
In order to purify HC at high engine power operating conditions and to decrease the amount of NOx exhausted into the atmosphere. Publication HEI 1-139145 further proposes to install a platinum (Pt)/alumina catalyst for oxidizing HC and CO included in the exhaust gas in a portion of the exhaust conduit located downstream of the Cu/zeolite catalyst. However, in this known apparatus, the ability of the Pt/alumina catalyst to purify whatever NOx has passed through the Cu/zeolite catalyst without being completely reduced by the Cu/zeolite catalyst is small.
The reasons why the NOx purification ability of the Pt/alumina catalyst in the Cu/zeolite-Pt/alumina exhaust gas purification system is small are thought to be as follows:
One reason is that the NOx which flows from the Cu/zeolite catalyst to the Pt/alumina catalyst is mainly in the form of NO, and the Pt/alumina catalyst has little capability for decomposing NO into N.sub.2 and O.sub.2. Another reason is that, in a high power condition, a relatively large amount of HC flows to the Pt/alumina catalyst. The HC will gradually degrades the catalyst and thus shortens its life.